A monomeric MnIII-peroxo complex derived directly from dioxygen

Article abstract of DOI:10.1021/ja802775e

The binding and activation of dioxygen by transition metal complexes is a fundamentally and practically important process in chemistry. Often the initial steps involve formation of peroxometal species that is difficult to observe because of their inherent reactivity. The interaction of dioxygen with a manganese(II) complex (1) of bis[(N-tert-butylurealy)-N-ethyl]-(6-pivalamido-2-pyridylmethyl)amine was investigated, leading to the detection of a new intermediate that is a peroxomanganese(III) complex (2). This complex is high-spin (S = 2) with a g value of 8.2 and D = -2.0(5) as determined by parallel-mode electron paramagnetic resonance spectroscopy. The coordination of a peroxo ligand was established using Fourier transform infrared spectroscopy that reveals a new signal at 885 cm^-1 for 2 when formed from ¹⁶O₂-this band shifts to 837 cm^-1 when ¹⁸O₂ is used in the preparation. Moreover, electrospray ionization mass spectra contain a strong ion at an m/z of 576.2703 for the ¹⁶O-isotopomer that shifts to 580.2794 in the ¹⁸O-isotopomer. Complex 2 also is capable of oxidatively deformylating aldehydes, which is a known reaction of peroxometal complexes. The similarities of 2 to the peroxo intermediates in cytochrome P450 are noted. Copyright

Full text of DOI:10.1021/ja802775e

Published on Web 06/21/2008
A Monomeric MnIII-Peroxo Complex Derived Directly from Dioxygen
Ryan L. Shook,† William A. Gunderson,‡ John Greaves,† Joseph W. Ziller,† Michael P. Hendrich,‡
and A. S. Borovik*,†
Department of Chemistry, UniVersity of CaliforniasIrVine, 1102 Natural Sciences II, IrVine, California 92697-2025,
and Department of Chemistry, Carnegie Mellon UniVersity, Pittsburgh, PennsylVania 15213
Received April 15, 2008; E-mail: aborovik@uci.edu
The binding and activation of dioxygen is an essential process
in synthetic and biological chemistry.1 The activation processes are
often proposed to involve formation of peroxometal complexes, as
exemplified by bleomycin and the mono-oxygenases cytochromes
P450.2 It is generally agreed that the initial steps in the O2 binding/
activation process in these enzymes involve a superoxoiron(III)
intermediate that converts to a hydroperoxoiron(III) species through
addition of an electron and proton. In this report, we demonstrate
that a similar O2 to peroxo conversion is operable in a synthetic
manganese system.
The observation of synthetic monomeric peroxometal complexes
is frequently difficult because of their inherent reactivity. This is
especially true for peroxomanganese complexes, where the MnIV2(µ-
1,2-peroxo) complex of Wieghardt is the only O2-derived system
that has been structurally characterized.3,4 Others have found that
treating MnII or MnIII complexes with superoxides5 or peroxides6
produce systems with monomeric peroxomanganese centers—this
approach has yielded a handful of complexes at low temperatures
that were stable enough to be characterized. We have been
investigating the interactions of dioxygen with manganese com-
plexes containing intramolecular hydrogen bonding (H-bond)
networks.7 Our systems utilize urea-based tripodal ligands that
provide H-bond donors to coordinated O-atom species. The MnII
complexes of these ligands bind and activate dioxygen producing
monomeric oxomanganese complexes.7,8 We have developed a
hybrid ligand (H5bupa) that combines two urea arms with one
carboxyamidopyridyl moiety9—the MnII complex of this ligand
binds O2 to produce a detectable peroxomanganese(III) species.
Preparation of the precursor 1 is outlined in Figure 1.10 Treating
H5bupa with 3 equiv of KH in dimethylacetamide (DMA) followed
by 1 equiv of Mn(OAc)2 afforded K[1] and 2 equiv of KOAc.
Figure 1. Preparative routes for 1 and 2, showing two possible tautomers
for 2. The thermal ellipsoid plot of [MnIIH2bupa]- is drawn at the 50%
probability level, and non-urea hydrogen atoms are omitted for clarity.
Selected distances (Å): Mn1-N1, 2.275(3); Mn1-N2, 2.214(3); Mn1-N3,
2.100(3); Mn1-N4, 2.125(3); Mn1-O1, 2.070(2).
which is converted to azobenzene (>95% yield). Monitoring the
reactions with optical spectroscopy shows that 2 has a visible
absorbance band at λmax ≈ 660 nm and a shoulder at 490 nm (Figure
S1).12 Similar spectra have been reported for MnIII complexes
containing a coordinated peroxo ligand.5d,6b
The oxygenation of 1 was followed by electron paramagnetic
resonance (EPR) spectroscopy (Figure S2). Perpendicular-mode
X-band EPR spectra of [MnIIH2bupa]- reveal the complex as a
nearly axial S ) 5/2 spin system with a large zero-field splitting
constant of D ∼ 0.3 cm-1. After exposure to O2, the S ) 5/2 signal
decreases as a new parallel-mode EPR signal associated with 2
appears at a g value of 8.2 (Figure 2A). A quantitative simulation
of the signal (Figure 2B) indicates an S ) 2 ground state with a
six-line (I ) 5/2) hyperfine splitting of a ) 57 G. Variable
temperature studies determined that the signal is from the ground
doublet with D ) -2.0(5) cm-1. The spin state, zero-field splitting,
and hyperfine constant are in agreement with other known mono-
meric MnIII species.5d,13 Moreover, the negative sign for the axial
zero-field splitting constant is consistent with tetragonally elongated
octahedral coordination geometry. The simulations also indicate
that 2 accounts for 80(10)% of the Mn in the sample. In
perpendicular-mode, this sample also showed the signal of the initial
Mn(II) complex (6%) and a mixed valent species at g ) 2 (4%).
The parallel-mode signal vanishes after prolonged incubation (6
h) at room temperature—the identity of the resultant species are
under investigation.
The molecular structure of 1 determined by X-ray diffraction
shows a five-coordinate MnII complex, having a distorted trigonal
bipyramidal geometry.10 The trigonal plane is defined by the
deprotonated urea and pyridyl nitrogen atoms of [H2bupa]3-; the
apical N1 atom and carbonyl oxygen O1 from the deprotonated
carboxamide occupy the axial positions. The remaining portions
of the urea groups form the scaffolding of a cavity, in which NH
groups are positioned inward toward atom O1. However, the
N6(N7)· · ·O1 distances are greater than 3.2 Å, distances that are
too long for intramolecular H-bonds.
A new green species (2) is formed in approximately 50% yield11
when [MnIIH2bupa]- reacts with O2 at room temperature. In DMA,
the reaction is relatively slow (∼30 min), yet the formation of 2
can be completed in approximately 10 min when 0.5 equiv of
diphenylhydrazine (DPH) is added to the reaction mixture (Figure
1). The yield of 2 also increases to nearly 80% when using DPH,
Isotopic labeling studies support the presence of a peroxo ligand
coordinated to the MnIII center in 2. Solution FTIR spectra recorded
at room temperature contained a peak at 885 cm-1 for 2 prepared
† University of CaliforniasIrvine.
‡ Carnegie Mellon University.
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8888 J. AM. CHEM. SOC. 2008, 130, 8888–8889
10.1021/ja802775e CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
external substrates, such as DPH, via a H-atom abstraction process
to initially produce a η1-hydroperoxoMn(III) complex (Figure 1,
2a). The reduction and protonation of the superoxo ligand mirrors
steps proposed during turnover in cytochrome P450. The tautomeric
η2-peroxomanganese(III) species (Figure 1, 2b) could be formed
from 2a by intramolecular proton transfer from the hydroperoxo
to the carboxamido component of the tripodal ligand. In this
pathway, the pivaloylamide moiety is re-formed to provide an
additional H-bond donor within the cavity. At present, we cannot
distinguish between these two structural possibilities. Nevertheless,
our findings establish that a mononuclear peroxoMn(III) can be
produced from O2 at room temperature.
Figure 2. Parallel-mode EPR spectrum (A) and simulation (B) of 2 (10
mM in DMF) recorded at 11 K. Microwave frequency and power, 9.379
GHz, 0.2 mW; modulation, 10 G. Simulation parameters: S ) 2, g ) 2.0,
D ) -2 cm-1, E/D ) 0.13(3), A ) 160 MHz.
Acknowledgment is made to the NIH (GM050781 to A.S.B.;
GM77387 to M.P.H.) for financial support.
Supporting Information Available: Experimental details for all
chemical reactions and figures for all spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
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Figure 3. FTIR (A) and negative-mode ESI-MS (B) spectra of 2 after
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under a 16O2 atmosphere (Figure 3A). The 18O-isotopomer can be
prepared from 18O2, causing a shift in the peak to 837 cm-1. The
observed vibrational change between the two isotopomers is as
expected based on a harmonic O-O oscillator (ν(16O2)/ν(18O2) )
1.06; calcd ) 1.07).14 These vibrational values are in the range
normally observed for other metal-based peroxo systems. For
instance, the η2-peroxoMnIII(Tp)15 complexes of Kitajima, formed
using H2O2, have FTIR-active peaks at 892 cm-1 that were assigned
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of 2 prepared with 16O2 exhibits a strong ion with a mass-to-charge
ratio (m/z) of 576.2703 (Figure 3B), a shift of 33 mass units from
the peak associated with 1 (Figure S3). The mass and calculated
isotopic distribution corresponds to the addition of a hydroperoxo
ligand to 1 (calcd, 576.2706; Figure S4A). Furthermore, when 2
was prepared from 18O2, the molecular ion peak shifts by 4 mass
units (Figure 3B) to a m/z of 580.2794 (calcd, 580.2792; Figure
S4B).
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(10) Full experimental details are found in Supporting Information.
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(12) The extinction coefficient for this peak is less than 300 M-1cm-1
.
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Preliminary reactivity studies indicate that 2 leads to the oxidative
deformylation of aldehydes. For instance, treating 2 with cyclo-
hexanecarboxaldehyde afforded cyclohexanone as the only GC-MS
detectable product in an unoptimized yield of 40% (eq 1). Note
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The spectroscopic, mass spectrometry, and reactivity results are
consistent with 2 being a monomeric peroxomanganese(III) com-
plex. A possible mechanism for its formation would involve a
superoxomanganese(III) intermediate that reacts with solvent or
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